This invention relates to an improvement in FM receivers and more specifically is directed to improving the audio sound quality by reducing the magnitude of audible pops that occur during input signal minimums caused by Rayleigh faded received signals. This invention is especially effective for FM systems which utilize peak deviations of less than 5.0 kHz.
Conventional FM receivers utilize sufficient intermediate frequency (IF) amplification so that the received signal as carried by the IF is in full "limiting" prior to a discriminator converting the FM information into audio. Limiting refers to the nonlinear amplification of the signal such as by an amplifier going into saturation or cut-off, or by a clipping circuit. The reason for utiliiing limiting is to remove amplitude signal variations prior to the recovery of the desired information by the discriminator because the discriminator is responsive to amplitude variations as well as frequency variations. Thus, limiting minimizes amplitude variations associated with the received signal. In conventional FM receivers, the total signal amplification or gain prior to recovery by the discriminator is normally set such that ambient noise in the passband of the receiver in the absence of desired RF signal is sufficient to cause limiting. Thus, it is apparent that a desired received signal which may be many decibels (dB) above the noise threshold will be in hard limiting.
In order to appreciate this invention the concept of a Rayleigh faded signal must be understood. Rayleigh fading refers to rapid fluctuations in the magnitude and/or phase of the received signal. A Rayleigh faded signal is most objectionable to a listener when the magnitude of a fade is great enough to cause a substantial momentary decrease in the signal-to-noise ratio. This results in the listener hearing an objectionable noise burst or "pop". A common example of Rayleigh fading occurs when a mobile radio user travels down a highway while receiving a signal having rapid and substantial magnitude fluctuations in field strength. Such fluctuations may be caused by out of phase reflections which result in field strength variations at the vehicle's mobile antenna.
The undesired audio responses due to Rayleigh fading become more objectionable as the peak transmitted deviation decreases and the carrier frequency increases. Deviation is a factor because it is related to the signal-to-noise ratio. In order to achieve the given level of audio output, more receiver gain will be required in a system having smaller peak deviation. Because this gain also amplifies the undesired noise burst, an audio pop is heard during a null produced by a Rayleigh fade. The pop is louder and hence more objectionable in a lower deviation system because more gain is utilized. Since wavelength decreases as frequency increases, a larger number of signal strength nulls exist to be encountered by a mobile radio operating at higher frequencies. Thus, at higher frequencies more frequent audio pops are likely.
Since the available frequency spectrum is limited and there continues to be an increasing demand for wireless communication channels, it is apparent that better utilization of the current available communication channels is desired. One way to better utilize the existing channel allocations is to divide the current channel bandwidths and provide more narrowband channels. For example, if an existing 25 kHz channel was divided in half, two 12.5 kHz channels could exist. Obviously narrower channels require that transmitted signals occupy less bandwidth. Decreasing the peak deviation from 5 kHz to 2.5 kHz in an FM system is one way to reduce the bandwidth of the transmitted signal in order to create more communication channels. New communication channels are becoming available at higher frequencies. Thus, the likelihood of having smaller deviation systems at higher frequencies makes the audio pop problem associated with Rayleigh fades a significant problem.